Migrate to cargo_embargo. am: 5af18216f9

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/syn/+/2854959

Change-Id: I662d907438cca110b2deb2f1c96cc850ddd5f5a6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>